Conjugation has been widely used to optimize the properties of biologically active proteins, such as protein therapies, antibody drug conjugates (ADCs), vaccines, tissue selective targeting vehicles, molecular diagnostics, and protein nucleic acid conjugates. Traditional conjugation method utilizes lysine based covalent ligation, which makes it difficult to achieve homogeneity due to the abundance of lysines on the protein's surface.
Site-specific labeling of proteins can be achieved by post-translational enzymatic reactions, for example, using human O6-alkylguanine-DNA alkyl-transferase (AGT), biotin ligase, transglutaminase, sortase, cutinase, or 4′-phosphopantetheinyl transferases for the covalent attachment of a label to a protein.
For post-translational enzymatic reactions using human O6-alkylguanine-DNA alkyl-transferase, the AGT is fused to a target protein of interest, followed by the addition of a labeled O6-benzylguanine, which is a suicide substrate for the AGT (Keppler et al., Nat. Biotechnol. 21:86-89, 2003). This approach is the basis for a technology called SNAP-Tag™, which utilizes a 180 amino acid tag (Tirat et al., International Journal of Biological Macromolecules, 39:66-76, 2006). However, labeling of proteins using this approach occurs only at the C- or N-termini.
For biotin ligation, the enzyme biotin protein ligase (BPL) attaches biotin to the biotin carrier domain of certain carboxylases or decarboxylases. BPL catalyzes, in a two-step, adenosine-5′-triphosphate (ATP)-dependent reaction, the post-translational formation of an amide bond between the carboxyl group of biotin and the ε-amino group of a specific lysine residue located within a highly conserved Ala-Met-Lys-Met (SEQ ID NO: 1017) recognition located motif within the biotin carrier domain (Tirat et al., International Journal of Biological Macromolecules, 39:66-76, 2006). This approach can be used to create fusion tags at the C-terminus, the N-terminus or even within the target protein and is the basis for a technology called BioEase™ (72 amino acid tag) and AviTag™ (uses the biotin ligase, BirA and 15-residue acceptor peptide tag (AP)).
Transglutaminases catalyze the formation of stable isopeptidic bonds between the side chains of glutamine (Gln) and lysine (Lys) with the loss of ammonia, and have been used to label glutamine side chains in proteins with fluorophores in vitro (Sato et al., Biochemistry 35:13072-13080, 1996). Also, bacterial and human tissue transglutaminases (BTGase and TG2) have been used to catalyze the post-translational modification of different IgG's via the Lys or Gln side chains located in the IgG heavy chain (Mindt et a, Bioconjugate Chem. 19:271-278, 2008; Jeger et at, Angew. Chem. Int. 49:9995-9997, 2010).
Sortases have been used for C-terminal and N-terminal site specific modification of proteins, where sortase A catalyzes the transpeptidation reaction (Antos et al., JACS, 131:10800-10801, 2009).
Cutinase is a 22-kDa serine esterase that forms a site-specific covalent adduct with phosphonate ligands that is resistant to hydrolysis. Cutinases have been used for C-terminal and N-terminal site specific modification of antibodies followed by immobilization onto surfaces (Kwon et al., Anal. Chem. 76:5713-5720, 2004; Hodneland et al., Proc. Natl. Acad. Sci. U.S.A., 99:5048-5052, 2002).
4′-Phosphopantetheinylation of acyl carrier proteins (ACPs) and peptidyl carrier proteins (PCPs) are involved in an essential post-translational modification that is required to activate metabolite synthesis by polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs), respectively (Fischbach et al., Chem. Rev. 106(8):3468-3496, 2006). The apo to holo conversion of ACPs and PCPs is catalyzed by 4′-phosphopantetheine (ppan) transferases, which attach a 4′-phospho-pantetheinyl moiety of coenzyme A (CoA) to an invariant serine residue of the protein domains (Lambalot et al., Chem. Biol. 3(11):923-936, 1996). Due to the comparably small size of the carrier proteins and the ability of 4′-phosphopantetheinyl transferases to accept functionalized CoA analogues as substrates, researchers have used carrier proteins as fusion tags to label target proteins with a variety of small molecule probes (see, e.g., La Clair et al., Chem. Biol. 11(2):195-201, 2004; Yin et al., J. Am. Chem. Soc. 126(25):7754-7755, 2004). In an effort to further reduce the carrier protein tag size, Walsh and co-workers used phage display to identify 8- to 12-residue peptides that are recognized as efficient substrates by the bacterial 4′-phosphopantetheinyl transferase Sfp (previously identified as a genetic locus responsible for surfactin production) and AcpS (Yin et al., Proc. Natl. Acad. Sci. USA 102(44):15815-15820, 2005; Zhou et al., ACS Chem. Biol. 2(5):337-346, 2007; Zhou et al., J. Am. Chem. Soc. 130(30):9925-9930, 2008).
Antibody drug conjugates (ADCs) have been used for the local delivery of cytotoxic agents in the treatment of cancer (see e.g., Lambert, Curr. Opinion In Pharmacology 5:543-549, 2005). ADCs allow targeted delivery of the drug moiety where maximum efficacy with minimal toxicity may be achieved. As more ADCs show promising clinical results, there is an increased need to develop stable engineered antibodies that provide reactive groups capable of conjugation to various agents, especially site-specific conjugations that can generate homogeneous immunoconjugates with a defined drug-to-antibody ratio for use in cancer therapy.